The present invention relates to a method of manufacturing a semiconductor device having a highly reliable groove isolation structure and a semiconductor device manufactured by the method.
An SGI (Shallow Groove Isolation) structure is one of the typical structures for electrically insulating adjacent elements such as transistors on a semiconductor substrate. This SGI structure is formed as follows At first, a pad oxide film and an oxidation inhibition film are formed ona silicon substrate. Then, a shallow groove is also formed on an insulating portion of the silicon substrate. Next, the oxide film is coated on the shallow groove by means of thermal oxidation and then an insulating film is coated on the oxide film.
The SGI structure is suitable for the device of 0.25 .mu.m process or smaller, because it has a higher precision in working dimension than the conventionally-used LOCOS structure.
However, this SGI structure has a problem that a silicon shape on an upper end of the groove may become acute in the thermal oxidation If the acute portion is left on the surface of the substrate, for example, as published by A. Bryant et al. in "Technical Digest of IEDM '94, pages 671 to 674, an electric field is concentrated on this acute portion while the circuit is in operation, so that the withstand characteristics of the transistors or the capacitors composing the circuit may be degraded.
It is experientially known that the degrade of the withstand characteristics takes place in the case that the curvature radius of the substrate around the upper end of the groove is 3 nm or lower even if the angle of the substrate around the upper end of the groove is 90 degrees or more. In order to solve these problems, as indicated in JP-A-2-260660, it is possible to form the pad oxide film on a lower portion so as to recede from the upper end of the groove by 0.1 .mu.m and perform the oxidation with a gas containing vapor at a temperature of around 1000.degree. C., for forming a curvature on the upper end of the groove.
As indicated in JP-A-6-21214, after forming a groove on the silicon substrate, amorphous silicon or epitaxial silicon is deposited to have a thickness of 5 nm or more. Then, the coat is oxidized at a temperature of 1100 to 1150.degree. C. and in the atmosphere of oxygen whose pressure is lowered by 20%. This is the indicated method of forming a curvature on the upper end of the groove.
In this method of depositing amorphous silicon, on the upper end of the groove, the amorphous silicon film is made thinner than the tabular portion of the substrate. Hence, the oxidation in this state preferentially makes the corner oxidized, so that the corner of the silicon substrate is made round.
Moreover, if the method of depositing the epitaxial silicon film is executed after forming the groove, as a result, the corner is chamfered so that the shape after oxidation is made round.